Pálacio da Pena (Pena Palace)
As you could read in the general description of Sintra, Pena Palace is the tourist hotspot of this town. To somewhat limit how crowded it gets inside, tickets for the palace are distributed for 30 minute timeslots. The time indicated on your ticket is the time you can enter the palace, NOT the time you can enter the park. The park can be visisted at any time, and cheaper tickets are available if you only want to visit the park without visiting the palace on the inside. However if you do have a ticket to visit the palace, bear in mind that it is located on top of a hill in the park. The journey from the entrance of the park to the entrance of the palace will take you about thirty minutes. This means that you have to enter the park at least thirty minutes before your timeslot for the palace to make it in time. Do not miss your timeslot!
Beat the crowds with an early timeslot. Early timeslots sold out first, especially in high season, book in advance. park open half an hour before palace.
Castelo dos Mouros
Not far from Sintra, quite the climb/hike up, surrounded by nature, break from tourist buses and busy village
Especially in the midst of the summer high season, I would consider the Moorish Castle worth your time. Do bear in mind that not only getting to the castle, but also following the narrow walls requires a lot of walking and climbing stairs on uneven ground. A basic fitness level is required. Furthermore, the views are one of the best aspects of the Moorish Castle. This means that going on a cloudy day might spoil half your fun. So make sure to check the weather forecast and plan your visist accordingly.
Voorbode of Physical fitness also in the castle itself
tickets at entrance, no reservation